Two-tone keyer



Patented Jan. 18, 1949 Eugene R. Shenk, Brooklyn,

N. Yi, assigner =`to' Radio Corporation of America,I afcorporation et v Delaware Application. March 16, 1946 Serial No. 655,004 l 9` Claims. 1;

Thisv application discloses a. new and improved method of and means for generating oscillatory energies of two frequencies alternatively present and representing respectively mark and space condition as used in frequency shift to telegraphy or black and white (contrast) as used in facsimile and similar picture signalling systems. The telegraphy or facsimile receiver systems may be of the type described in Schock et al. application Serial #632,978', filed December 5, 1945, whereinis shown a receiver of such signals.

In many transmission systems where it is necessary to distinguish between two senses, for example, in` frequency shift telegraphy signals or in black and white picture signals the two-tone or frequency shift method of signalling, is becoming widely accepted as most` desirable. This is in part because in the two-tone system energyis present in` the receiver during spacing condition as well as during marking condition so that a more positive control of the recording apparatus is had. v

Heretofore in two-tone keying systems, keying has in many cases been accomplished by providing continuously both tone frequencies and then by use of either a mechanical or electronic switch selectively choosing between the two tones in accordance with signal energies. A disadvantage of these known systems. resides in the fact that as switching takes place from one tone to the other tone, parasitic oscillations and transients are generated in the circuits and then are transmitted with the signals unless elaborate filtering and damping means are used;

A general object of thepresent invention is to provide new and improved means of accomplishing two-tone keying of the nature described above.

A more specific object of the present invention is to provide an improved method of and means for generating oscillatory energies of differenttone frequencies separated in the frequency spectrum the desired and rigidly fixed amount and for selectively translating said tone. frequencies alternatively in accordance with signals for signalling purposes.

The above objects are attained in accordance with my invention by application of the Well known fact thatthe harmonic frequency content of a rectangular wave is `a function of the ratio of the positive portion of` the wave to. the total period of the Wave. More speciflca1ly, in accordance with my invention-the. ratiooi the positive to the negative portion. of. a rectangular alternat. ingcurrent waveY is `so.selecte`d aste .render 4the amplitude r of a `given harmonic.zerowhilefmaximizing another harmonic. The above ratio is then changed by signal `keying so vas to maximize the first harmonic and to minimize the said sec'- ond harmonic.` Suitable-filtering circuits are then arranged toaccept only the desired frequencies, one representing marking condition and the other representing spacing condition, or one representing black, the other representing white in a picture-system, The terms positive and negative portions as applied to the waves are used broadly and include excursions inopposite directions irrespective of the sign of the voltage.

In `describing myiinvention in detail reference will-be made to the `detailed drawings wherein Fig. 1 illustratesby block diagram and schematically the essential features of a two-tone keyer arrangedv in accordance with my invention.

Fig. 2 illustrates byl -circuit element including electron discharge devices, and circuit element connection, the essential features of a two-tone keyer arranged in accordance with my invention. In this embodiment the details of the selecting or filtering circuits are-not shown since no claim to the same per se is being made herein.

Fig. 2a illustrates inV a table form operation of the frequency dividerused4 in the embodiment ofr Figs. 1 and 2; while Fig. 3 illustrates by wave voltagel diagram the manner in which the `harmonics are generated and selected in the arrangement of Fig. 2.

In Fig. 1 rectangle I represents a` source of oscillatory energy of any desired type which is of. say sine wave formI and of substantially constant frequency f. Rectangle 2 includes an amplier and a current amplitude limiter `and wave form controller which is excited byoscillatory energy from sourceV I` of the' frequency f. The oscillations of frequency fare the synchronizing voltage, and are supplied to "rectangle, 3 wherein is included a frequency dividing multivibrator controlled, by' oscillatory energy of the frequency f,

to operate at a. controlled frequency F. Rectangles; 4 vand 5 includingfllters for selecting harm'oni'c energy of different frequencies`r representing dilferent signalling condition and supplying-the, same to a two-tone output line from which the two tones may be impressed on atransmitting system. The two tones are separated by a fixed frequency band. and since they are alternatively present they may be suppliedover the common output circuit. The output of. the multivibrators in 3 is supplied ir parallel to thetone iilters in 4 and 5,

one of which accepts a frequency, say nF, and the other of which acceptsa frequency mF.

l As will. be seenj from the. descriptionwhich follows, the arrangement is such that when marking voltage is supplied at the keying input terminals to control the amplitude of the wave form out of unit 2, the output wave form of the multivibrator in unit 3 is also controlled and is such that the frequency component nF is zero, but the component of frequency mF is considerable. Likewise, when the keying input to unit 2 is a voltage representing spacing condition, the amplitude of the synchronizing wave supplied to unit 3 is changed and the wave form of the multivibrator output is modified in such a manner that mF becomes zero and a large component of nF is present. It should be noted that the output tone frequencies are synchronous and are in the frequency ratio n/m. Furthermore, the frequency separation between the tones is an integral multiple of the fundamental frequency multivibrator in unit 3.

Details of the essential features of my system, except for the filters, has been shown in Fig. 2. In Fig. 2 the tubes V6 and V1 are in a multivibrator circuit which is, except as otherwise noted hereinafter, conventional. The control grids of the tubes are connected to the cathodes and ground by biasing resistors. The anodes and control grids are cross-coupled and the anodes are connected to the positive terminal of a source E of direct current potential through resistance including potentiometer resistors Pl and P2. The periodicity of the operation of the multivibrator including tubes VB and V1 depends in part at least upon the magnitude of the anode potential. The output of the multivibrator is supplied tothe control grid of a coupling tube V8 the anode of which is resistively coupled in parallel to the two filters 4 and 5. The source I may be any appropriate source of oscillations of substantially constant frequency and may be of Asine wave form. The output of this source is supplied to an amplifier tube VI which is biased by cathode resistor I0 and anode resistor I1 to operate as an amplifying current amplitude limiter. The output of this amplifier is fed to the grids of tubes V2 and V3 in parallel. A decoupling resistor I3 is included in the coupling to the grid of tube V3. The tubes V2 and V3 are similarly operated as current amplifiers and current amplitude limiters. Limiting takes place in the tubes VI', V2 and V3 by virtue of the fact thatl the control grids are driven beyond cutoff by the oscillatory energy on the negative halves of the cycles and are driven to saturation by the oscillatory energy on the positive halves of these cycles. The cathode circuit of tube V2 includes biasing resistor R2 of considerable value, while the anode circuit thereof includes anode load and voltage dropping resistor RI. The cathode circuit of tube V3 is similarly raised above ground by a biasing resistor R4 and its anode circuit includes an impedance and voltage dropping resistor R5. The anode of tube V2 is coupled to a point on potentiometer PI by condenser l5 while the anode of tube V3 is coupled to a point on potentiometer P2 by condenser I8. The tube V2 then supplies synchronizing voltage to the plate circuit of tube V5 and the magnitude of this synchronizing voltage is adjustable by means of potentiometer PI. 'I'he tube V3 supplies synchronizing voltage to the anode circuit of tube V1 by way of condenser I8 and the magnitude of this vltage is adjustable by means of potentiometer P v 4'Ihe signal Apotential which inthe example given may be considered to be zero voltage in the wir,

4 presence of marking condition or in the presence of black, and may be highly negative in the presence of spacing condition or when the picture is to be white, is applied in parallel to the control grids of two tubes V4 and V5. A decoupling resistor i9 is included in the coupling to the control grid of tube V5. The tube V4 is connected in shunt to the resistor R2 by the resistor R3. When tube V4 is non-conducting the anode cur rent of tube V2 all flows through resistor R2 and considerable voltage is lost therein and the gain of this tube falls as does the magnitude of the synchronizing voltage fed to potentiometer PI. The tube V5 has its anode connected by resistor i 20 to the anode of tube V3 so that this tube draws anode current through resistor R5, thereby sharing the current flow through this resistor R5 and the potential drop at the anode end thereof with the tube V5. Therefore, when tube V5 is nonconductive the .gain of the tube V3 is greater than when the tube V5 is made conductive by the signals.

In the statement of operation which follows it is assumed that the source I supplies output of 1800 cycles per second. Other frequencies may be used. However, it is preferable that the frequency of the source l be a multiple of the frequency of operation of the multivibrator.

When zero voltage is applied to the input of tubes V5 and V4, potentiometers PI and P2 are so adjusted that both tubes V5 and V'i divide by the saine factor, in the example given by 3. This is shown in the chart of Fig, 2a. Thus the overall "J5 or total division of the multivibrator stage is 6.'

In this -condition the output wave shape from the multivibrator is referred to asa 50:50 Waveform, and it is positive for 50 per cent of the time and negative for the other 50 per cent of the time as illustrated in Fig. 3. A Fourier analysis of such a wave form reveals that the fourth harmonic component is zero amplitude. This fourth harmonic is 1200 cycles per second. The fundamental frequency of the multivibrator is 300 cycles per second, i. e., 1800 cycles divided by 6. Consequently, no output is obtained from the 1200 cycle per` second lter in unit 4. The 50:50 waveform of Fig. 3, however, does contain a strong componentof third harmonic frequency, which i in this case is 3 times 300 or 900 cycles per second. Therefore, lthe 900 cycles per second or marking filter supplies 900 cycles per second frequency output tothe two-tone line. Under these conditions it has been assumed that the tubes V4 and V5 have been conductive so that the bias on the cathode `of tube V2 is reduced by the low impedance of tube V4 in shunt thereto and this tube is of 'relatively high gain while the potential at the anode of vtubeV3 has been reduced by current flow through R6 to supply tube V5 and this tube is Aof relatively small gain.` Then the adjustment is made `as statedl above by means of potentiometers Pl and P2 so that both multivibrators have a factor of division of 3, or a total of 6, and the wave form is weighted 50:50.

The values of the component parts associated with tubes V4 and.V5 are further selected so that when these tubes are rendered non-conductive by application tothe control grids thereof, of a highly negative voltage representing say spacing condition in frequency shift signalling o'rwhite in picture signalling, `the amplitude of the synchronizing voltage supplied by tube V2 over condenser I6 toA potentiometer PI and the anode of tube V6 falls sufficiently so that its orderY of division lincreases to 4. This amplification This 4Weight l is' now `33 1/3 5662/3. two Aparts of a total of six -parts are positive, and

`spacing condition, etc.

falls byfvlrtue offthe ifactlth'attheitube VAL becomesinon-'conductive and the bias on the feathuode of the tube V2 increases to lowerthe gain oi this tube. l Ina somewhat like manner and simultaneously the potential on the anode `of tubeVB l `increases so that the synchronizingvoltage suppliedby 'the tube"V3 over condenser `-I8Mto the potentiometer P2 in the anode circuit of rV'liin- 'creases `to aneXtent-such as to cause the tube V1 to divide by 2. `As a resultfthe` total-order of *division of the multivibrator remains unchanged,

thatuisto say,` the` multivibrator as` a Wholeldivides l by 6- and this output stays at` afundamentalfrequency F30() cycles -persecond. "How- `ever, the Wave form -is no longer of `50`z50fweight. Inother `vvords,

`to the output circuit or line.

Whlie the keying input voltage inFig. 2 has been identified as zero voltage in Vmarking condition anda highly negative voltage in spacing condition, where the marking and spacing terminology has been arbitrarily carried over from the on and off type of telegraphy, it will be understood that zero voltage may represent Furthermore, this keying input might equallywell represent picture signals Where, for examplethe black 'portions might be represented by Zerovoltage and the White `parts by negative voltage.

While only one numerical example and schematic circuit diagram have been shown to illus- "f tratesmeans for realizing thef invention, it Will be apparent to anyone skilledl inthe `art that by proper design -any-setiof `tonefrequencies can readily be obtained and that other detailed circuitvcon'iigurations Will accomplish the same result as doesthat of Fig. 2. A

" What is claimedis:

l. In a signalling system, a source of alternating voltage of substantially iixed frequency the durations of the alternations of which, above and below respectively a base value, are of a first ratio, a source of signalling potentials the magnitude of which changes, means coupled to said source of alternating voltage and controlled by said signal potentials for changing said ratio, and Wave filters, arranged -to select alternating' voltages of different frequencies, coupled to said source of alternating voltage.

2. In a signalling system, a source of alternating voltage of substantially fixed fundamental frequency the durations of the alternations of which, above and below respectively a base value, are of a iirst ratio, said voltage being rich in components of a first harmonic frequency, a source of signalling potentials the magnitude of which changes, means coupled to said source of alternating voltage and controlled by said signal potentials for changing said ratio so that said voltage is rich in components of a second harmonic frequency, and tWo Wave iilters, one tuned to said first harmonic and the other to said second harmonic, coupled to said source of alternating voltage.

3. In a signalling system, a frequency divider including two tubes in an oscillation generating 'circuit controlledby oscillating voltages x to oper- -ate at a fundamental!frequency, .the frequency at which each tube of thefdivider tends :topperu .ate depending in party on the' tube circuit-.iconstants and the magnitude vof theucontrolling voltages, said divider --operating to produce oscillatory energyof afwave form whereinitheratio ofthe duration of theexcursionsvof the oscillations'sin one direction to the' duration of the'excursions of theoscillations in the otherldirection-ishof a first value,` means for varying-the magnitude of `the voltages controlling# the` frequency divider `in accordance Withsignals to changesaid ratio to a second value, and iilterstunedito different'harmonies .of vsaid fundamentaldrequency "coupled to'fsaid `frequency divider.

il; In a telegraphy` or similai-` systemy a. frequency divider includingftwo tubes in an oscillation, generating circuit, a-source of oscillating voltages `of substantially fixed frequency coupled thereto to `entrain the frequencyfdividerfor operation at its fundamental frequency, the frequency atwhich each tube of the 'dividerrtends to operatebeing dependent inf part on-the tube circuit constants and the magnitude *of therapplied controlling voltages, .said divider as controlled operating to4 produce oscillatory energy of rectangular wave form wherein the ratio of the duration of the Wave excursions in one direction to the duration of the wave excursions yin the other directionlis-of a rst value, a source of potentials of varying magnitude representing signals operating on said coupling between said first source and said divider to vary the magnitude of the applied controlling voltages to thereby vary said ratio, and iilters` tuned to different `harmonics of said fundamental' frequency, coupled tosaid divider.

5; In a telegraphy or similarA system,iafrequency divider `including two tubes in; an oscillation generating circuit, a sourceof oscillating voltages of substantially fixed frequency coupled thereto to entrain the frequency divider for oper ation at its fundamental frequency, the irequenu cy at which each tube of thedivider tends to operate being dependent in part on the tube circuit constants and the magnitude oi the applied controlling voltages, said divider as controlled operating to produce oscillatory energy of rectangular wave form Wherein the ratio ofthe duration of the Wave excursions in oneY direction to the duration of the Wave excursions in the other direction is of a rst value, and wherein the oscillatory energy has large components of one harmonic of said fundamental and is deiicient in components of another harmonic of said fundamental, a source of potentials of varying magnitude representing signals operating on said coupling between said iirst source and said divider to vary the magnitude of the applied controlling voltages to thereby vary said ratio, and two filters, one tuned to said one harmonic frequency and the other to said other harmonic frequency, coupled to said divider.

6. In a signalling system, a frequency divider comprising a pair of electron discharge tubes having electrodes interconnected by capacitors and connected to potential sources by resistors of such a value that the divider operates to generate oscillations of a fundamental frequency, a sour-ce of oscillations of a frequency which is a multiple of the frequency at which said divider operates, an amplifier tube in a iirst path coupling said source to a resistor in the circuit of said divider, a second amplier tube in a second path coupling one direction to the length of the excursions in the other direction is of a first ratio, a source of signal potentials, means for differentially modulating the gain of said ampliiier tubes in accordance With said signals to generate oscillatory voltages in said divider the length of the eX-A cursions of which in said one direction to the length of the excursions vin the other direction is of a second ratio, and frequency selective circuits coupled to said divider.

7. In a wave frequency reducing system, a source of oscillatory energy, a frequency divider coupled to said source and controlled thereby to generate a voltage of rectangular wave form of substantially fixed fundamental frequency which is a submultiple of the frequency of the oscillations of said source and which includes a strong component which is a harmonic of said fundamental, of one order, means coupled to said divider to select output energy of said one order harmonic frequency, a source of control potentials,

means for modifying the magnitude of the firstmentioned oscillatory energy controlling said di- 'viderin accordance with said control potentials to change the relative lengths of the excursions about the axis of said wave of rectangular form,

so that the same includes a Strong component which is a harmonic of said fundamental of another order, and means coupled to said divider to select energy of said other order harmonic frequency.

8. In a wave frequency reducing system, a source of oscillatory energy, a frequency divider coupled to said source and controlled thereby to generate a voltage of rectangular wave form of substantially fixed fundamental frequency which is a submultiple of the frequency of the oscillations of said source and which includes a strong component which is a harmonic of said fundamental, said voltage being decient in components of another harmonic frequency, means,

coupled to said divider to select output energy of said one harmonic frequency, a source of control potentials, ineans for modifying the magnitude of the first mentioned oscillatory energy controlling said divider in accordance with said control potentials to change the relative lengths of the excursions about the axis of said wave of rectangular form so that the same includes a strong component at said other harmonic of said fundamental frequency but is deficient in` components of the frequency of said one harmonic frequency, and means coupled to said divider to select energy -of said other harmonic frequency. y Y Y 9. In a frequency shift telegraphy system, a multivibrator comprising a pair of electron discharge tubes having electrodes interconnected by capacitors and connected to potential sources by resistors of such a `value that the multivibrator operates to generate oscillations of rectangular Wave form, a source of oscillations of a frequency which is a multiple of the fundamental frequency at which said multivibrator operates, a current amplitude limiter tube having input electrodes coupled to said source of oscillations, a iirst amplier tube in a nrst path coupling the output of saidycurrent amplitude limiter tube to a resistor in the circuit of one of said multivibrator tubes, asecond amplifier tube in a second path coupling the output of said current limiter tube to a resistor in the circuit of the other multivibrator tube, to entrain the multivibrator to operate at said fundamental frequency, which is a submultiple of the frequency of said source, to generate oscillatory voltages the length of the excursions of which in one direction to the length of the eX cursions in the other direction is of a rst ratio, a source of signal potentials, means for differentially modulating the Aconductivity of said ampliertubes in accordance with signals to change said ratio, and two lters tuned to different harmonies of said fundamental frequency coupled to said multivibrator.

EUGENE R. SHENK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,159,596 Miller May 23, 1939 2,338,395 BartelinlsV Jan. 4, 1944 2,392,114 Bartelink Jan, 1, 1946 2,413,932 Sziklai Jan. 7, 1947 2,418,268 Lawson Apr. 1, 1947 

